/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.log4j;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Hashtable;
import org.apache.log4j.helpers.Loader;
import org.apache.log4j.helpers.ThreadLocalMap;

/**
 * The MDC class is similar to the {@link NDC} class except that it is based on
 * a map instead of a stack. It provides <em>mapped
   diagnostic contexts</em>. A
 * <em>Mapped Diagnostic Context</em>, or MDC in short, is an instrument for
 * distinguishing interleaved log output from different sources. Log output is
 * typically interleaved when a server handles multiple clients
 * near-simultaneously.
 * 
 * <p>
 * <b><em>The MDC is managed on a per thread basis</em></b>. A child thread
 * automatically inherits a <em>copy</em> of the mapped diagnostic context of
 * its parent.
 * 
 * <p>
 * The MDC class requires JDK 1.2 or above. Under JDK 1.1 the MDC will always
 * return empty values but otherwise will not affect or harm your application.
 * 
 * @since 1.2
 * @author Ceki G&uuml;lc&uuml;
 */
public class MDC {

	final static MDC mdc = new MDC();

	static final int HT_SIZE = 7;

	boolean java1;

	Object tlm;

	private Method removeMethod;

	private MDC() {
		java1 = Loader.isJava1();
		if (!java1) {
			tlm = new ThreadLocalMap();
		}

		try {
			removeMethod = ThreadLocal.class.getMethod("remove", null);
		} catch (NoSuchMethodException e) {
			// don't do anything - java prior 1.5
		}
	}

	/**
	 * Put a context value (the <code>o</code> parameter) as identified with the
	 * <code>key</code> parameter into the current thread's context map.
	 * 
	 * <p>
	 * If the current thread does not have a context map it is created as a side
	 * effect.
	 */
	static public void put(String key, Object o) {
		if (mdc != null) {
			mdc.put0(key, o);
		}
	}

	/**
	 * Get the context identified by the <code>key</code> parameter.
	 * 
	 * <p>
	 * This method has no side effects.
	 */
	static public Object get(String key) {
		if (mdc != null) {
			return mdc.get0(key);
		}
		return null;
	}

	/**
	 * Remove the the context identified by the <code>key</code> parameter.
	 */
	static public void remove(String key) {
		if (mdc != null) {
			mdc.remove0(key);
		}
	}

	/**
	 * Get the current thread's MDC as a hashtable. This method is intended to
	 * be used internally.
	 * */
	public static Hashtable getContext() {
		if (mdc != null) {
			return mdc.getContext0();
		} else {
			return null;
		}
	}

	/**
	 * Remove all values from the MDC.
	 * 
	 * @since 1.2.16
	 */
	public static void clear() {
		if (mdc != null) {
			mdc.clear0();
		}
	}

	private void put0(String key, Object o) {
		if (java1 || tlm == null) {
			return;
		} else {
			Hashtable ht = (Hashtable) ((ThreadLocalMap) tlm).get();
			if (ht == null) {
				ht = new Hashtable(HT_SIZE);
				((ThreadLocalMap) tlm).set(ht);
			}
			ht.put(key, o);
		}
	}

	private Object get0(String key) {
		if (java1 || tlm == null) {
			return null;
		} else {
			Hashtable ht = (Hashtable) ((ThreadLocalMap) tlm).get();
			if (ht != null && key != null) {
				return ht.get(key);
			} else {
				return null;
			}
		}
	}

	private void remove0(String key) {
		if (!java1 && tlm != null) {
			Hashtable ht = (Hashtable) ((ThreadLocalMap) tlm).get();
			if (ht != null) {
				ht.remove(key);
				// clean up if this was the last key
				if (ht.isEmpty()) {
					clear0();
				}
			}
		}
	}

	private Hashtable getContext0() {
		if (java1 || tlm == null) {
			return null;
		} else {
			return (Hashtable) ((ThreadLocalMap) tlm).get();
		}
	}

	private void clear0() {
		if (!java1 && tlm != null) {
			Hashtable ht = (Hashtable) ((ThreadLocalMap) tlm).get();
			if (ht != null) {
				ht.clear();
			}
			if (removeMethod != null) {
				// java 1.3/1.4 does not have remove - will suffer from a memory
				// leak
				try {
					removeMethod.invoke(tlm, null);
				} catch (IllegalAccessException e) {
					// should not happen
				} catch (InvocationTargetException e) {
					// should not happen
				}
			}
		}
	}

}
